spring webflux kotlin CoroutineW

CoroutineWebFilter.kt:

package com.example.webflux.filter

import kotlinx.coroutines.reactive.awaitSingleOrNull
import kotlinx.coroutines.reactor.mono
import org.springframework.web.server.ServerWebExchange
import org.springframework.web.server.WebFilter
import org.springframework.web.server.WebFilterChain
import reactor.core.publisher.Mono

abstract class CoroutineWebFilter(
    /**
     * true - only execute once during one request; false - execute every time even if the request was forwarded by handler A to handler B
     */
    private val once: Boolean = false
) : WebFilter {
    private val processFlag = "__FILTER_PROCESSED_" + javaClass.name

    final override fun filter(exchange: ServerWebExchange, chain: WebFilterChain): Mono<Void> {
        val flag = if (once) exchange.getAttribute<Any>(processFlag) else null
        return if (flag == null) mono {
            if (beforeChain(exchange)) {
                chain.filter(exchange).awaitSingleOrNull()
                afterChain(exchange)
            }
            null
        } else Mono.empty()
    }

    /**
     * @return true - continue to next step; false - skip all remain steps and return to client
     */
    abstract suspend fun beforeChain(exchange: ServerWebExchange): Boolean

    abstract suspend fun afterChain(exchange: ServerWebExchange)
}